Although OFDM communication is known and used in earth-based communication systems using higher order (hierarchical) modulation, it presents a problem for transmission of such hierarchically modulated signals from an earth-orbiting satellite to an earth based receiver. The OFDM modulation scheme has a high peak-to-average power ratio, due to the number of active carriers that are transmitted simultaneously. For power amplifiers, it is highly desirable to run the output at or near saturation, since this typically maximizes the power added efficiency: RF output power/DC power. However, running at saturation poses a problem for OFDM systems, because the higher peaks tend to clip; and any clipping in time causes a distortion over all the active carriers. Thus, typically, a terrestrial system will use a 6 dB back-off from saturation to allow only strong peaks to get clipped. But this reduces broadcast signal strength, and thus coverage area. To improve coverage area in land-based systems, more repeaters are used.
The solution of more repeaters is not very practical for satellite-based transmitters, since the cost of a satellite is very high, and having the maximum RF output power is thus desired. There have been proposals to operate satellite-based OFDM transmitters with a 3 dB back-off from saturation, but this smaller back-off causes significantly more distortion than the traditional 6 dB back-off. To compensate for this smaller back-off, a more powerful forward error correcting (FEC) code (Turbo or LDPC) can be used. This works well for traditional signals but poses a problem for hierarchical modulation of the type wherein a primary (high priority) signal is modulated by a secondary (low priority) signal of lower power and the resulting modulated signal itself modulates the carrier(s). Such hierarchical modulation allows for various levels of performance in the overall system. However, when the power amplifier is backed off too much from saturation, the higher power (primary) signal is clipped; and this causes distortion equally on both the high power primary signal and the lower power secondary signal. This clipping appears as additional noise that can render the secondary signal unrecoverable by a receiver.
It is known in the prior art that one may decrease the data transmission rate of the secondary data modulated onto the primary data in hierarchical modulation by providing known instances where the secondary modulation signal has no energy, but this process of the prior art does not optimally address the problem of clipping in hierarchically modulated OFDM communications when operating the transmitter power amplifier close to saturation. Clipping of a hierarchically modulated OFDM sample creates noise that is spread across all the OFDM carriers and can completely obscure the low priority (secondary) data in the symbol. An optimal solution concentrates on reducing the clipping rather than reducing the low priority data rate relative to the high priority data rate.